Old percussion rifle

Question
I have an old percussion rifle that has a beatiful curly maple stalk. It has a 33.5 inch barrel which carries the only visible marking of H. Johnston Great Western Gun Works, Pittsburgh Pa.
it has one hammer and two triggers which i found odd(probably due to my lack of knowledge with these types of firearms). The butt of the gun has a conforming curl made of brass and a little decorative compartment built into the stalk located on the right side. is there any information you can give regarding this weapons age or approximate value or where I may locate this information? Thank you in advance for your assistance!

Answer
Hi,
This is all I can find for you
http://books.google.com/books?id=tFuN-JNoOhkC&pg=PA244&lpg=PA244&dq=H.+Johnson+G...

I do not have black powder era reference materials,
Sorry I can't help more.